Manually regulated vehicle heating and air conditioning systems include an operator interface for enabling or disabling air conditioning, and for selecting airflow mode (i.e., panel, defrost, etc.), blower speed, and discharge air temperature. When air conditioning is enabled, a refrigerant compressor is activated and its capacity controlled based on a system temperature or pressure to maintain the evaporator temperature a few degrees above the freezing point of water to provide maximum cooling and dehumidification while preventing evaporator icing. The discharge air temperature is selected by positioning a temperature control lever or knob, which in turn, positions a discharge temperature control mechanism (such as an air mixing door) to direct all or a portion of the air exiting the evaporator through a heater core coupled to the engine coolant circulation system. In typical operation in warm ambient conditions, the driver will enable air conditioning, and set the blower speed to high and the temperature to full cold. As the vehicle cabin cools down, the driver usually lowers the blower speed and adjusts the temperature lever until a desired combination of discharge air flow and temperature is achieved. In this scenario, the compressor control remains essentially unchanged, and a portion of the air exiting the evaporator is re-heated by the heater core to achieve the desired air discharge temperature.
It has been recognized that the efficiency of the above-described control can be improved by regulating the capacity of the compressor as the temperature selector is moved away from the full cold setting. In this way, the evaporator temperature is allowed to increase above the full cold setting, and the need for re-heating to achieve the desired air discharge temperature is reduced. For example, the U.S. Pat. No. 4,383,574 discloses a control wherein movement of the temperature lever in the cold-to-medium portion of the temperature control range changes the capacity of the refrigerant compressor while an air mixing device is maintained in the full-cold position, and movement of the temperature lever in the medium-to-hot portion of the temperature control range changes the position of the air mixing device to heat the discharge air. While this system can achieve improved efficiency when air conditioning is enabled by lowering the compressor input power requirement, it also creates an undesired temperature control non-linearity when air conditioning is disabled, since movement of the temperature lever in the cold-to-medium portion of the temperature control range will produce no change in the discharge air temperature. Accordingly, what is needed is a coordinated control of the compressor capacity and the discharge temperature control mechanism that provides improved efficiency when air conditioning is enabled without disturbing the temperature control linearity when air conditioning is disabled.